The present invention pertains to corrosion inhibition of treated and untreated aluminum flake pigments alone and for use in waterborne coating compositions. Aluminum flake pigments are slightly reactive in an aqueous environment having a basic pH, such as aqueous coating compositions. The pH of the water-borne acrylic coating compositions typically ranges from 8.0-9.0, and the polyurethane coating compositions have a pH typically ranging from 7.5 to 8.0.
In a basic pH environment, the aluminum pigment reacts with water to produce hydrogen gas and aluminum hydroxide. The reaction is a form of corrosion and converts the pigment to a hydrated oxide form unsuitable for pigment use, as it destroys the metallic pigmentation properties of the mirrorlike particles. The amount of corrosion is measured by the amount of hydrogen gas produced over a given period of time.
Once formation of aluminum hydroxide begins, corrosion accelerates. This is because the OH.sup.- ions cause the pH of the environment to further increase, resulting in the increased formation of aluminum hydroxide.
The aluminum pigment deteriorates over time due to oxidation, as it remains in continuous contact with the basic pH environment of the coating composition. Coating compositions containing the pigment are often stored for 6 months or more before application, which can result in significant corrosion of the pigment. If this corrosion remains unchecked the coating composition may be unusable.
An additional factor affecting degradation of the pigment in a coating occurs when the paint is pumped from the storage site to the application site. This distance is often several blocks in length. The pumping action required to transport the paint this considerable distance is severe and results in breaking of the pigment particles, which exposes even more of the surface area of the pigment to corrosion. Corrosion inhibition can be achieved if the rates of these reactions, or any partial steps involved, can be decreased.
It has been found that time released corrosion inhibitor compounds inhibit corrosion in aluminum flake pigment particles in a basic aqueous environment. The corrosion inhibitor compound is the reaction product of water-soluble compound of yttrium or rare earth metal and an anionic metal complex including compounds selected from the group consisting of soluble salts of transition metal oxo-complexes complexes, soluble silicate salts and mixtures thereof.
It has further been discovered that aluminum flake pigment treated with the corrosion inhibitor compound is highly effective to inhibit the corrosion of the pigment particles in water-borne coating compositions. This is significant because, as explained above, the physical factors such as pumping and extended storage of coating compositions expose the pigment to increased corrosion at the high pH of water-borne coating systems. Use of the treated pigment significantly reduces the corrosion and/or degradation of aluminum flake pigment used in metallic coatings.